1. Field of the Invention
The present invention relates to quadraphase modulators, and more particularly to a wideband quadraphase modulator having a high phase accuracy.
2. Description of the Prior Art
Typical quadraphase modulators accept an input signal and may be controlled to produce one of four outputs, the phases of the outputs being in quadrature. Known modulators of this type suffer from a loss in power and in phase accuracy. The output may vary in both phase accuracy and power with frequency.
The following U.S. patents typify the prior art: U.S. Pat. Nos. 4,736,170 to Wu et al.; 4,276,521 to Davidheiser; 4,673,898 to Redmond; and 4,021,758 to Standing. These references disclose the use of a 90.degree. hybrid and a 0.degree. hybrid combination to produce quadrature outputs. A reversing switch is required such as a double balanced mixer or biphase modulator. 90.degree. hybrids depend for operation on the coupling between two circuits. However, the coupling is less at the passband edges of the device and the signal level will vary between output ports, the difference being referred to as the imbalance of the hybrid.
When a 90.degree. hybrid is used with a 0.degree. hybrid, the amplitude imbalance produces a phase imbalance. This problem occurs since each output of the quadraphase modulator is a resultant of two vectors at 90.degree. to one another. The imbalance causes the vectors to be unequal, thereby producing a phase error in the resultant.
There is therefore a need for a quadrature modulator utilizing hybrids that is free from phase error due to imbalance and that will have improved bandwidth and frequency response.